an unexpected property of the cosmos-that it was expanding more slowly in
the past. Or in other words, the expansion is accelerating. Skeptics suggest the
dimming could merely be the result of intervening cosmic dust or other dis
tortions or of some intrinsic change in the nature of supernovae over the eons.
But team members say that their own rigorous efforts to knock their own find
ings down, based on those same suspicions, have so far proven futile. Their
discovery was named Science magazine's "Breakthrough of the Year" for 1998.
An accelerating expansion would assist in bringing the age of the universe
into accord with the ages of its oldest stars: If the universe was expanding
more slowly in the past, it took longer to reach its current stage than is
implied by measurements of the current expansion rate-and therefore
could be older than it looks.
In order to accommodate the new evidence of an accelerating universe,
some scientists have turned to a fudge factor known as the cosmological
constant. This perennial angel has
been invoked over the years to bail
out problem equations. A concept
that Einstein introduced and later
dismissed as a mistake, it proposes
an exotic repulsive energy that fills
"empty" space, separating objects
rather than drawing them together as
ordinary gravity does. Scientists who
study the smallest units of nature
have learned that this vacuum is
actually seething with activity at the
subatomic levels as particles pop in
and out of nothingness. Though
these are virtual particles, laboratory
NASAAND ROBERTWI
experiments have shown that they
have real and predictable effects. Such particles may be exerting an influence
that accumulates across vast distances in a way that is accelerating the expan
sion of the universe.
"The cosmological constant is good news for inflation theorists," says Alan
Guth, "since most versions of the inflation theory call for a density of matter
and energy that flattens the universe." So far astronomers have not found that
much matter, but the invisible energy associated with the cosmological con
stant could be just what is needed. This energy would allow scientists, in
essence, to have their cake and eat it too. Now they can have a low-density
universe that, thanks to the mysterious energy in the vacuum, is also flat and
effectively eternal. The equations work out.
THENEW FINDINGS, though, are tied up with another frustration.
Scientists have long admitted, with some chagrin, that they are
unable to locate most of the mass in the universe-the sources of
gravity. The evidence for this missing mass comes from two sources:
One is the actual observed motions of galaxies, gases, and stars, which would
fly apart from each other if there were not some influence much more power
ful than their own gravity holding them together. The other comes from
inflation theory and its requirement for just the density needed to keep the
universe flat.
Scientists have looked high and low for the missing mass. Is it in ghostly
dark galaxies, roving collapsed objects, strange particles of energy such as the
UNVEILING THE UNIVERSE